This invention relates to multi-mode radar systems and particularly to an improved airborne radar system which can be used to selectively map weather conditions or interrogate navigation beacons and display the responses thereto.
Airborne weather radar systems installed in aircraft are known which operate in the X-band of microwave frequency and which have means for transmitting a radar pulse into space and for receiving and displaying the radar energy reflected from weather cells such as clouds, rain and other forms of precipitation.
It is desirable that the radar gain be adjusted by the automatic gain control (AGC) circuits during each period between successive radar pulse transmissions. This interpulse period gain adjustment must, of course, be accomplished after all radar returns of interest have been received and displayed by the radar system, that is, after targets within the maximum range of interest have been displayed.
Since the purpose of the gain adjustment is to keep noise at a constant level it is important that the adjustment be made during the time that no signal return of interest is being intercepted by the radar system. However, returns from targets beyond the maximum range of interest will continue to be intercepted by the radar system even though such target returns are not to be displayed on the system display. In the present embodiment, rather than spacing successive radar pulses at relatively long intervals in order to permit the receiver gain adjustment to be made only after the radar returns have been effectively attenuated into the noise, a side step generator is provided which side steps the local oscillator to thereby detune the radar receiver out of the expected radar return frequency so that after radar returns from the maximum range of interest have been intercepted the receiver is off-tuned or side stepped to a free portion of the frequency spectrum and the gain then adjusted.
It is also known that the weather radar transmitter frequency can be selected by the system designer to be equal to the standard radar ground beacon interrogation frequency and the weather radar thereafter used in a navigational mode to interrogate such ground beacons. Ground beacon responses to such interrogations are pulse space coded and transmitted at a predetermined frequency which is somewhat different from the interrogation frequency. The present embodiment shows how, with a minimum of hardware, the airborne multi-mode radar can be used to receive and process not only weather returns but also beacon replies merely by referring the radar system local oscillator to the radar system magnetron and providing a plurality of selectable IF amplifiers.